Butyl rubber is known for its excellent insulating and gas barrier properties. Generally, commercial butyl polymer is prepared in a low temperature cationic polymerization process using Lewis acid-type catalysts, of which a typical example is aluminum trichloride. The process used most extensively employs methyl chloride as the diluent for the reaction mixture and the polymerization is conducted at temperatures on the order of less than −90° C., resulting in production of a polymer in a slurry of the diluent. Alternatively, it is possible to produce the polymer in a diluent which acts as a solvent for the polymer (e.g., hydrocarbons such as pentanes, hexanes, heptanes and the like). The product polymer may be recovered using conventional techniques in the rubber manufacturing industry.
In many of its applications, butyl rubber is used in the form of cured compounds. Vulcanizing systems usually utilized for butyl rubber include sulfur, quinoids, resins, sulfur donors and low-sulfur high performance vulcanization accelerators.
Peroxide curable butyl rubber compounds offer several advantages over conventional, sulfur-curing systems. Typically, these compounds display extremely fast cure rates and the resulting cured articles tend to possess excellent heat resistance. In addition, peroxide-curable formulations are considered to be “clean” in that they do not contain any extractable inorganic impurities (e.g. sulfur). The clean rubber articles can therefore be used, for example, in condenser caps, biomedical devices, pharmaceutical devices (stoppers in medicine-containing vials, plungers in syringes) and possibly in seals for fuel cells.
It is well accepted that polyisobutylene and butyl rubber decomposes under the action of organic peroxides. Furthermore, U.S. Pat. No. 3,862,265 and U.S. Pat. No. 4,749,505 teach us that copolymers of a C4 to C7 isomonoolefin with up to 10 wt. % isoprene or up to 20 wt. % para-alkylstyrene undergo a molecular weight decrease when subjected to high shear mixing. This effect is enhanced in the presence of free radical initiators.
One approach to obtaining a peroxide-curable butyl-based formulation lies in the use of conventional butyl rubber in conjunction with a vinyl aromatic compound like divinylbenzne (DVB) and an organic peroxide (see JP-A-107738/1994). In place of DVB, an electron-withdrawing group-containing a polyfunctional monomer (ethylene dimethacrylate, trimethylolpropane triacrylate, N,N′-m-phenylene dimaleimide) can also be used (see JP-A-172547/1994).
The disadvantage of these methods is that the resulting compound is contaminated with the low molecular weight reagents added to induce crosslinking, which did not fully react with the rubber in the solid state. Also, the action of peroxide on the regular butyl rubber may lead to formation of some low molecular weight compounds from the degraded rubber. The final articles based on such compounds may display an undesirable characteristic of leaching out the said low molecular species and accelerated aging.
A commercially available terpolymer based on IB, IP, and DVB, Bayer XL-10000, is curable with peroxides alone. While said commercial pre-crosslinked polymers exhibit excellent properties in many applications, they have a gel content of at least 50 wt. % which sometimes makes the even dispersion of fillers and curatives normally used during vulcanization difficult. This increases the likelihood of under- and over-cured areas within the rubbery article, rendering its physical properties inferior and unpredictable. Also, the Mooney viscosity of this rubber is high, usually 60–70 units (1′+8′@125° C.) which may cause significant processing difficulties, during mixing and sheeting stages.
Co-Pending Canadian Application CA-2,316,741 discloses terpolymers of isobutylene, isoprene and DVB prepared in the presence of a chain-transfer agent, such as diisobutylene, which are substantially gel-free and have an improved processability. Co-Pending Canadian Application CA-2,386,628 discloses peroxide curing and high purity applications for said terpolymers of isobutylene, isoprene and DVB prepared in the presence of a chain-transfer agent, such as diisobutylene. However, the presence of free DVB can present serious safety concerns. For these reasons, it would be desirable to have an isobutylene based polymer which is peroxide curable, completely soluble (i.e. gel free) and does not contain any DVB in its composition.
White et al. (U.S. Pat. No. 5,578,682) teach a process for obtaining a polymer composition comprising polymeric material selected from the group consisting of polypropylene, copolymers of propylene with up to 10 mole % ethylene, polyisobutylene, copolymers of a C4 to C7 isomonoolefin with up to 10 wt % isoprene, copolymers of a C4 to C7 isomonoolefin with up to 20 wt % of para-alkylstyrene and mixtures thereof, said polymeric material having a number average molecular weight in the range of from about 5,000 up to 5,000,000, and characterized as having a bimodal molecular weight distribution containing a lower molecular weight species and a higher molecular weight species, said higher molecular weight species comprising at least about 1.0 weight percent of said polymeric material. The polymer was mixed with a polyunsaturated crosslinking agent (and, optionally, a free radical initiator) and subjected to high shearing mixing conditions in the presence of organic peroxide. This patent is silent about an isoolefin-multiolefin-PAS (PAS=para-alkylstyrene) terpolymer and any filled compounds of modified polymers or the cure state of such compounds.
The present invention describes the preparation of butyl-based, peroxide-curable compounds which employ the use of an isoolefin-multiolefin-PAS terpolymer in conjunction with a peroxide initiator and co-agent. The present invention clearly demonstrates that there is an advantage associated with using a material which contains both a multiolefin and PAS in the polymer backbone.